A gripper cylinder, which has one or several grippers on its shell face, which grippers are movable between a position in which they keep a leading edge of a piece of flat material, which is to be conveyed on the gripper cylinder, pressed against the shell face, and a release position, in which the flat material can be released again, or a fresh piece of flat material can be picked up and clamped, is generally known. The grippers typically perform a pivot movement between these two positions. Since the periods of time available for accomplishing such clamping or releasing of a product are short, the pivot movement must be performed at a high speed. The movement amplitude between the clamping position and the release position of the gripper should be as small as possible in order to keep strong accelerations, which stress the material, within limits.
To prevent damage to a trailing end of a piece of flat material, which trailing end is maintained on the cylinder by a gripper, and which possible damage may be caused by the movement of a gripper which, in the course of clamping a following piece of flat material, which piece of flat material follows in the circumferential direction of the cylinder, most gripper cylinders are laid out for picking up pieces of flat material which are fed to the gripper cylinder spaced apart from each other. The spaced apart pieces of flat material respectively each come to rest against the gripper cylinder while forming a gap between successive pieces. The gripper can thus move in the gap, without touching the respectively previous piece. If these pieces of flat material were previously produced by being cut off a continuous web, in order to form such a gap, the cut-off pieces must be accelerated to a speed which is greater than that of the continuous web, prior to its being cut. However, if a conveying system, which conveys the products cut off the continuous web further after such cutting, runs faster than the fed-in continuous web, this results in slippage. Such slippage results in friction between the conveying system and a leading section of the continuous web penetrating it which continuous web, prior to its being cut off, necessarily still moves at the original speed of the continuous web of which it is a part. In connection with flat material which have a sensitive surface, such as, for example freshly printed products, this friction can impair the quality of the surface, for example by the imposition of drag marks or of smudging of the ink. Moreover, if the pieces of flat material are put together from a stack of sheets, which sheets are not connected with each other, the problem arises that different friction between different sides of the stack can lead to the sheets being displaced with respect to each other, and the stack being pulled apart. Such displacement makes the further processing of the stack considerably more difficult.
It is particularly problematical if the pieces of flat material are cut off from the continuous web when these pieces are in direct contact with the gripper cylinder, such as, for example, by the use of a rotating cutter cylinder which, together with the gripper cylinder, defines a cutting gap, and which rotating cutting cylinder severs the continuous web while working together with a counterthrust element of the gripper cylinder. To insure that the continuous web to be cut is placed evenly against the surface of the gripper cylinder, the gripper must be capable of being lowered into, or of dipping into the interior of the gripper cylinder. After a piece of flat material has been cut off from the fed-in continuous web, there is only a brief period of time which is available for accomplishing the gripping of a freshly formed leading edge of the continuous web, by the use of a gripper, and the pressing of this freshly formed leading edge against the surface of the cylinder. However, the path between the lowered position of the gripper and the extended position of the gripper, in which extended position the gripper presses the flat material against the cylinder, is long and requires a high speed of the gripper movement, which high speed of movement can only be realized by the use of a high quality expensive drive mechanism. Moreover, increasing wear, and therefore susceptibility to failure, of the drive mechanism becomes higher, the greater the operating speed becomes.
A gripper cylinder is known from EP 0 931 748 B1 and from DE 198 57 507 A1, which is capable of conveying printed products, that are cut off a fed-in continuous web, without a lead, i.e. without a space between the successive printed products. In this gripper cylinder, a gripper is mounted on a shaft, which shaft is pivotably seated, via a translation mechanism, in the cylinder, and which drives the gripper, that is coupled to the pivot movement, to perform a parallel displacement. This translation mechanism is used to displace the gripper between its lowered position and a position in which the gripper is projecting past the shell face of the cylinder, from which projecting position, the gripper can be pivoted around the shaft in order to press the leading edge of a continuous web of printed products against the cylinder surface.
DE 100 60 713 A1, U.S. Pat. Nos. 6,093,139 A and 953,063 A each describes a folding apparatus with a gripper cylinder having a cutter strip for engagement by a cutter of a cutter cylinder, which is working together with the gripper cylinder. The grippers of the gripper cylinder are pivotable, around a first, movable shaft, for clamping the material to be transported. This first shaft is, in turn, seated on a lever, which lever is pivotable around a second shaft that is fixed in place on the cylinder. This lever is controlled, with the aid of a first cam disk, for providing a movement of the gripper in the cylinder circumferential direction. A second cam disk controls the clamping movement of the gripper.
DE 102 03 059 A1 discloses a transport cylinder with a gripper for use in transporting sheets of imprinted material. The gripper can be moved by the use of two pivot shafts, which are fixed in place on the cylinder, and by a further pivot shaft, which further pivot shaft is pivotable around one of the pivot shafts that is fixed in place on the cylinder. Only one cam disk is provided for control.
U.S. Pat. Nos. 5,429,578 and 5,004,451 show folding blade cylinders which work together with tapes for accomplishing the guidance of products.